blood samples are commonly withdrawn from the vein rather than the artery. Yet the content of venous blood can be quite different from the content of arterial blood in a subject at a given time and it is the blood in the arteries that is carrying alcohol into the brain, resulting in intoxication.

In the circulatory system, venous blood is blood returning to the heart. With one exception (the pulmonary vein) this blood is deoxygenated and high in carbon dioxide, having released oxygen and absorbed CO2 in the tissues. It is also typically warmer than arterial blood, has a lower p H, has lower concentrations of glucose and other nutrients, and has higher concentrations of urea and other waste products.